For cryocooler-based superconducting magnet systems, an efficient heat transfer between coils and cryocooler is crucial for their successful cooldown and reliable operation. Compared to conventional thermal buses generally made of highly conducting metals such as high-purity copper, the Pulsating Heat Pipe (PHP) offers high thermal performance at a reduced mass by utilizing a two-phase flow combining latent and sensible heat transfer mechanisms: Inside a capillary bent into serpentines between an evaporator (in contact with the coil) and a condenser (in contact with the cryocooler), the cryogenic fluid transfers heat through self-induced oscillations and overall circulation of a train of vapor plugs and liquid slugs.
In the framework of a collaboration between the Paul Scherrer Institute (PSI) and the VDL Enabling Technologies Group (VDL ETG), PHPs were experimentally characterized using helium, neon and nitrogen as working fluids for the cooling of cryocooler-based superconducting magnets in accelerators and industrial applications. Due to their specific saturation properties, these three fluids allow the operation at different temperature ranges between 2.2 to 126.2 K. However, the inner diameter of the capillary, necessary for the stable operation of PHPs and affecting their thermal resistance and maximum heat load preventing dry-out, depends on the fluid properties and is by common convention limited by a Bond number inferior to 4. For the application to both low and high temperature superconducting (LTS and HTS) systems, it is of interest to use different fluids in the same device or to use the optimal inner diameter of capillary for one fluid. Therefore, a parametric study was conducted to investigate the influence of the inner diameter- regardless of the Bond number criterion- on the performance of helium, neon and nitrogen PHPs.
This contribution briefly introduces the test setup, before concentrating on the results. The impact on PHP applications for the cooling of coils and current leads made of HTS or LTS is discussed for applications in science and industry.

Keywords
Pulsating heat pipe|Experiment|Two-phase flow|Cryogenics|Fluids